In recent years, laser light has been used in various applications, e.g., in the cutting and working of metals, as a light source in the photolithographic apparatus used in semiconductor manufacturing systems, in various types of measuring devices, and in treatment apparatuses and operations performed in the fields of surgery, ophthalmology and dentistry, etc. Recently, the treatment of myopia, hyperopia and astigmatism by the correction of the curvature or indentations and projections of the cornea through ablation of surface of the cornea by irradiation with laser light (PRK) or ablation of the interior of the cornea in which an incision has been made (LASIK) has received particular attention, and such treatment has seen some practical use. An apparatus in which ablation of the surface of the cornea is accomplished by irradiating the cornea with ArF excimer laser light (wavelength: 193 nm) is known as such a cornea treatment apparatus (for example, see Japanese Patent No. 2809959, Japanese Patent Application Kokoku No. H7-121268 and Japanese Patent Application Kokai No. H5-220189).
Ablation of the surface of the cornea using ArF excimer laser light utilizes the fact that the photons with a wavelength of 193 nm that make up ArF laser light have an energy that cuts material bonds such as C—N, C—C, C—O, C—H and C═C bonds, so that such light can break down peptides, which are the basic units of proteins. In this technique, ablation of the surface of the cornea is accomplished by irradiating the surface of the cornea with laser light so that the peptides are broken down and volatilized.
In this case, however, it is necessary to perform precise volatilization in which there is no heat-solidified layer in order to maintain the transparency of the cornea. For this reason, ArF excimer laser light at a wavelength of 193 nm is utilized at which mainly volatilization by the scission of material bonds occurs, with little thermal volatilization taking place. Furthermore, it is also possible to cause volatilization of the surface of the cornea by means of KrF laser light with a longer wavelength (248 nm) or XeCl laser light (wavelength: 308 nm). In such cases, however, the rate of occurrence of thermal volatilization is high in addition to volatilization effected by the scission of material bonds, so that a heat-solidified layer tends to be generated.
Furthermore, the following problem also arises: namely, the DNA inside cells is susceptible to damage when subjected to irradiation with laser light, so that there is a danger that spontaneous mutations will be induced. The absorption spectrum of DNA tends to show larger values at shorter wavelengths of light in the ultraviolet region; however, the effective induction of spontaneous mutations is determined by the quantity of light that passes through the cytoplasm and reaches the nuclei, as well as the absorption of light by the nuclei. The induction of spontaneous mutations is extremely great at wavelengths of 240 to 280 nm, and is lower at wavelengths that are shorter or longer than this range. Especially on the short wavelength side of this range, the absorption of light by the cytoplasm increases abruptly with a decrease in the wavelength. Accordingly, there is an abrupt decrease in the amount of light that reaches the nucleus as the wavelength becomes shorter, so that in the case of light at a wavelength of 193 nm, the amount of light that reaches the nucleus is almost zero. Furthermore, the following problem also arises: namely, light at wavelengths shorter than 193 nm is absorbed during propagation through air, so that the propagation efficiency is low. Judging from such facts, it appears that ArF excimer laser light with a wavelength of 193 nm is most suitable for use in corneal therapy, rather than KrF laser light or XeCl laser light.
However, an ArF excimer laser oscillating apparatus is constructed by sealing argon gas, fluorine gas and neon gas, etc., inside a chamber, and these gases must be tightly sealed. Furthermore, replenishment and recovery of the respective gases must also be performed, so that the apparatus tends to become large and complicated. Furthermore, in order to maintain a specified laser light generating performance in an ArF excimer laser oscillating apparatus, there is a need for periodic overhaul and replacement of the internal gases.